The invention relates to a negative-acting, radiation-curable composition comprising a compound that forms an acid under the action of high-energy radiation and an acid-hardening compound.
In conventional UV-lithography, the limit of resolution is predetermined by the wavelength of the radiation employed. Increasing miniaturization of dimensions in the fabrication of chips therefore requires new lithographic techniques in the submicron range, electron beams or X-rays being used in the process because of their extremely short wavelengths. It has been found that resist materials which are suitable for use as electron-beam resists can also be employed as X-ray resists, and vice versa.
Resist materials customarily used for this application comprise acrylates and methacrylates [G. M. Taylor, "Solid State Technology", 124 (1984)]. In these materials, sensitivity and pattern resolution have, in most cases, proved to be incongruous properties. In order to obtain relatively high sensitivities, halogens are usually incorporated in the resist. In the case of positive-acting resists, fluorine and chlorine are generally employed, whereas negative-acting resists preferably contain bromine and iodine, apart from chlorine [T. Yamaoka et al., Phot. Sci. Eng. 23, 196 (1979)].
Negative-acting, i.e., radiation-curable resists generally show a higher sensitivity than positive-acting resists. However, as explained above, they cannot have a high resolution in the submicron range at the same time. Positive-acting resists based on methacrylate, on the other hand, yield a high resolution but, with the exception of the resists based on polymethacrylonitrile, they do not withstand the plasma-etching processes used for the patterning of semiconductors. The methacrylate resists, for their part, have an insufficient sensitivity.
Polyalkenesulfones, in particular polybutene-1-sulfone, are the polymers with the highest sensitivity to electron beams and X-rays so far known. It is, however, a disadvantage of compounds of this category that they have a reduced resistance to plasma etching processes; they can thus be used for the production of masks, but they are unsuitable for use in the fabrication of semiconductors with the aid of masks made of this material. It has therefore been suggested to combine polyalkenesulfones with novolac resins that are plasma-etch resistant, as is generally known [M. J. Bowden et al., J. Electrochem. Soc., 128, 1304 (1981); U.S. Pat. No. 4,289,845]. Unfortunately, however, the two polymers showed a severe incompatibility, which impaired resolution; and when it was attempted to improve compatibility by admixing further components, a loss of sensitivity had to be accepted (U.S. Pat. No. 4,398,001).
To maintain high sensitivities within the context of improved general characteristics, in particular, improved plasma-etch resistance, photocatalytically acting resists were developed. Examples of positive-acting systems of this type are described, inter alia, in DE-A-27 18 254, 29 28 636 and 38 21 585.
Corresponding negative-acting systems comprise, for example, resists which, upon irradiation, are crosslinked in a dimerizing reaction, such as polymeric cinnamic acid derivatives and polyacrylates. Resists of this type are relatively insensitive. When the principle of photocatalysis is utilized in negative-acting systems, a difference is made between resists that can be photopolymerized by free radicals and resists which, due to photoinduced reactions, are crosslinked cationically, by addition, substitution or condensation. The first-mentioned resists exhibit considerable drawbacks in the imaging quality. Their applicability to uses in the submicrometer range is therefore limited.
The application of acid-hardening resins in photoresist formulations has been known for a long time. As described, for example, in U.S. Pat. No. 3,692,560 halogen-containing benzophenones in combination with melamine resins or urea-formaldehyde resins can be used as UV resists. DE-A-27 18 259 (=U.S. Pat. No. 4,189,323) describes the use of halogenated derivatives of s-triazine as photolytically activatable acid donors for positive and negative-acting systems, for example, for acid-hardening urea-formaldehyde, melamine-formaldehyde and phenol-formaldehyde resins (column 5). Examples of formulations of this kind are also specified by Vollenbroek et al., Microelectronic Engineering 6, 467, (1987).
It is also possible to use onium salts, such as diphenyl iodonium salts of non-nucleophilic acids, e.g., of HSbF.sub.6, HAsF.sub.6 or HPF.sub.6, as photolytically activatable acid formers. DE-A-27 30 725 describes the use of starters of this kind in resist formulations containing epoxides as acid-hardening materials. A general account of the use of onium salts in acid-hardening systems is given by J. V. Crivello, Polym. Eng. Sci. 23, 953 (1983).
EP-A-0 164 248 discloses photo-curable compositions on a basis of acid-hardening resins and naphthoquinone diazides or o-nitrobenzoic acid derivatives.
In EP-A-0 232 972 halogenated compounds, such as DDT or .gamma.-hexachlorocyclohexane, are described, which show virtually no absorption above 299 nm and are used as photoinitiators in acid-hardening negative photoresists.
In all systems described above the initiator compounds do not participate in the actual crosslinking process and are themselves insoluble in aqueous-alkaline developers, with the exception of the onium salts. This leads to drastically reduced development rates in those areas, where the non-exposed resist layer is to be removed by the developer. The amount of starter compound which can be added, and consequently also the sensitivity of the resist prepared, are thus clearly limited. The use of onium salts and also the use of the initiators described in EP-A-0 232 972, which are also known as plant protection agents, are moreover critical from a physiological standpoint.